The invention relates to open circuit, demand type breathing apparatus as used by divers, particularly but not exclusively, for use in contaminated waters.
A common type of breathing apparatus is the open circuit, demand type which uses a demand regulator fitted to a rigid mask frame and flexible hood assembly (termed "bandmasks"). Alternatively the demand regulator can be fitted to a hard helmet assembly. This type of breathing apparatus operates on "demand"--when the diver breathes in breathing gas is drawn in through an inhale port on the demand regulator into an oral/nasal mask cavity, and when the diver breathes out the exhaled gas passes out through an exhale port directly into the ambient water. The exhale port is fitted with a non-return valve to prevent exhaled gas or water being drawn in when the diver inhales. These known types of equipment, however, have the disadvantage that they cannot totally prevent ingestion of water by the diver. Water ingress occurs during the inhalation phase of the diver's breathing cycle, when water droplets are sucked past the exhale non-return valve in the demand regulator body and thence into the diver's lung via the oral/nasal mask cavity. This can be dangerous if the diver is in contaminated water and is undesirable even in uncontaminated conditions. The exhale non-return valve is the prime leakage path into the mask.
There are alternative types of breathing apparatus available which deal with the problem of possible contamination in different ways. One type is the closed circuit system in which possible contamination is prevented by sealing the system from the ouside water. Another type is the open circuit, positive pressure system in which possible contamination is avoided by keeping an excess pressure of gas in the mask to prevent contaminating material entering the mask. Neither of these ways of preventing contamination are applicable to the open circuit, demand type of breathing apparatus.